1. Field of the Invention
The present invention relates to an improvement in a film depositing method and film depositing apparatus for continuously forming large-area, functional deposited films by generating a uniform microwave plasma over a large area and decomposing and exciting raw-material gases by the reaction caused thereby.
More particularly, the present invention concerns a method and apparatus for forming a large-area photovoltaic element utilizing an amorphous semiconductor and, specifically, a means arranged to control a temperature of walls of a deposited film forming chamber, for obtaining a functional deposited film of good quality.
2. Related Background Art
Proposed as one of efficient mass-producing methods of photovoltaic elements is a method for fabricating an amorphous-silicon-based solar battery wherein independent film-forming chambers are provided for forming respective semiconductor layers of a solar battery. The semiconductor layers are formed in each of the corresponding film-forming chambers.
For example, the specification of U.S. Pat. No. 4,400,409 discloses the continuous plasma CVD system employing the roll-to-roll method. This system can continuously form an element having a semiconductor junction in such a way that a plurality of glow discharge regions are. A flexible substrate having a desired width and length is disposed along a path which passes through the respective glow discharge regions. The substrate is continuously conveyed in its longitudinal direction while necessary electroconductive semiconductor layers are deposited in the respective glow discharge regions.
Using microwaves as energy for generating a plasma is known. Since the wavelengths of microwaves are short, the energy density can be enhanced over the conventional cases using RF. Thus microwaves are suitable for efficient generation and continuation of plasma.
For example, U.S. Pat. No. 4,517,223 and U.S. Pat. No. 4,504,518 disclose a method for depositing a thin film on a small-area substrate in a microwave glow discharge plasma under low pressure. This method can obtain a high-quality deposited film and prevent polymerization of active species that could cause degradation of film characteristics. The method can outstandingly improve the deposition rate, because the process is under low pressure.
Further, the specification of U.S. Pat. No. 4,729,314 discloses the low-pressure microwave plasma enhanced CVD process and system for depositing a photoconductive semiconductor thin film on a large-area cylindrical substrate by a high-power process using a pair of radiation waveguide applicators.
Taking the above circumstances into consideration, a mass-producing method of higher throughput can be obtained by combining the microwave plasma enhanced CVD process (hereinafter referred to as ".mu.W-CVD process") with the roll-to-roll production method said to be suitable for mass production.
Let us consider, an example for fabricating an a-SiGe single-layer cell (single cell) solar battery using an a-SiGe layer for the i-layer (photoelectric conversion layer) by the roll-to-roll .mu.W plasma enhanced CVD process (hereinafter referred to as "R-R.mu.WCVD process"), the combination of the two foregoing methods.
A manufacturing apparatus by the R-R.mu.WCVD process is arranged to continuously deliver a beltlike substrate for formation of film of a-SiGe from a bobbin with a rolled substrate thereon, to form a plurality of layers including at least an n-type a-Si layer, an i-type a-SiGe layer, a p-type a-Si layer, and the like. The solar battery is formed in respective film-forming chambers (which are the same as the foregoing "deposition chambers") each being separate reactors. Connecting members (normally called "gas gates" or simply "gates") allow the substrate to move between the plural film-forming chambers while maintaining a reduced pressure condition in each film-forming space. They also prevent gases supplied to the respective film-forming chambers, for example, raw materials for the n-type a-Si layer, p-type a-Si layer, etc. from diffusing and mixing into each other.
FIG. 8 is a schematic drawing to show an apparatus for fabricating the semiconductor element of a-SiGe solar battery or the like by the R-R.mu.WCVD process, in which the .mu.W method is used to make the i-type a-SiGe layer having a large deposited film thickness which is required to be formed at a high throughput. The RF method is used to make the n-type and p-type a-Si layers having a small deposited film thicknesses and is not required to be formed at such a high throughput as compared with the i-type a-SiGe layer.
In FIG. 8, reference numeral 801 designates a beltlike substrate (hereinafter referred to simply as a substrate) for the a-Si layers to be deposited thereon. The substrate 801 is usually a deformable electroconductive substrate, for example, a thin plate of stainless steel, aluminum, or the like, or a member obtained by coating a non-conductive thin plate with a conductive thin film or the like. The substrate 801 is rolled around a circular bobbin 811, which is installed in a feed chamber 810. The substrate 801 sent out from the bobbin installed in the feed chamber 810 passes through gas gate (hereinafter referred to simply as "gate") 820, n-type a-Si film-forming chamber 830, gate 840, i-type a-SiGe film-forming chamber 850, gate 860, p-type a-Si film-forming chamber 870, and gate 880 to be wound up around a winding bobbin 891 installed in a winding chamber 890.
Each of 830a and 870a denotes an RF power supply, and each of 830b and 870b is a cathode electrode for exciting RF discharge, to which power for depositing the n-type a-Si layer or the p-type a-Si layer, respectively, is supplied.
Further, 850a is an applicator comprised of a dielectric window for radiating the microwave into the film-forming space, to which power is applied from a microwave power supply (not shown) through a rectangular waveguide tube 850b set perpendicularly to the dielectric window, thereby causing glow discharge in the discharge space in the i-type a-SiGe film-forming chamber.
Reservoirs 802a to 806a are each filled raw material gas for forming each deposited film, wherein 802a is filled with SiH.sub.4 gas, 803a with GeH.sub.4 gas, 804a with H.sub.2 gas, 805a with PH.sub.3 gas, and 806a with BF.sub.3 gas.
Each gas is guided through switch valve 802b to 806b and pressure reducing device 802c to 806c to gas mixer 830c, 850c, 870c.
A raw-material gas, adjusted at desired flow rate and mixture ratio in the gas mixer 830c to 870c, passes through gas inlet line 830d, 850d, 870d to flow into each film-forming chamber. The gas introduced into the film-forming chamber is evacuated to achieve the desired pressure in each chamber, by evacuation apparatus 810e, 830e, 850e, 870e, 890e comprised of an oil diffusion pump, a mechanical booster pump, and a rotary pump, or the like, to be guided to an unrepresented exhaust gas processing apparatus. Further, each of 830f, 850f, 870f is a heater for heating the substrate, to which power is supplied from power supply 830g, 850g, 870g, respectively.
Numeral 841 or 861 is a part for adjusting the cross section of the aperture of the gate, which reduces mutual diffusion of gas between the film-forming chambers by narrowing the gas flow path.
Further, a gas that does not negatively affect film formation, for example H.sub.2, He, or the like, is supplied through a gas inlet port 842 or 862 to the gate from a gas bomb 807a via a pressure reducing device 807b and a flow-rate adjuster 807c, 807d, thereby further suppressing mutual diffusion of raw-material gas in each film-forming chamber.
The substrate 801 (the "beltlike substrate" will be referred to as "substrate") sent out from the feed chamber 810 successively advances in each film-forming chamber, whereby the n-type a-Si film, i-type a-SiGe film, and p-type a-Si film are formed on the surface thereof, and then the substrate goes finally into the winding chamber 890.
First, the substrate 801 is heated up to a desired temperature by the heater 830f in the n-type a-Si film-forming chamber 830.
The gas mixer 830c mixes gases of SiH.sub.4, H.sub.2, PH.sub.3, and the like being the raw materials for the n-type a-Si film each at an optimum flow rate and the mixture is then introduced to the film-forming chamber 830. At the same time, the RF power is supplied from the RF power supply 830a to the cathode 830b to cause glow discharge in the film-forming space, thereby forming the n-type a-Si film on the surface of substrate 801.
Next, the substrate advances in the gate 840 and then goes into the i-type a-SiGe film-forming chamber 850. In the film-forming chamber 850, similarly as described above, optimum power is supplied to SiH.sub.4, GeH.sub.4, and H.sub.2 gases each set at an optimum flow rate, thereby forming the desired i-type a-SiGe film on the foregoing n-type a-Si film. Then the substrate 801 passes through the gate 860 and p-type a-Si film-forming chamber 870 in the same manner to be wound up around the bobbin 891 in the winding chamber 890.
Since the substrate is successively guided through the n-type, i-type, and p-type film-forming chambers in this way, the fabricating apparatus of the roll-to-roll method can achieve a very high throughput. (Problems of R-R.mu.WCVD)
(1) The problem with conventional R-R.mu.WCVD process is that the input microwave power is not used only for decomposition of raw-material gas for deposition of film, but the microwave power also indirectly heats the walls of the deposition chamber forming the film-forming space through the high plasma density or the microwave itself directly heats the walls to high temperatures.
The temperature of the walls of deposition chamber starts increasing at the same time as the input of microwave power and, after a while, reaches a saturation temperature determined by the discharge power value or the like at that time. The temperature may reach 300.degree. C. or even about 450.degree. C. depending upon the conditions.
The first resulting problem is that the temperature of the beltlike substrate increases as affected by the high temperature of the walls of deposition chamber, so that the substrate temperature cannot be maintained around 300.degree. C., which is normally considered to form a deposition film with good quality.
Solar batteries fabricated under such circumstances will have low photoelectric conversion efficiency.
(2) Depending upon the material for the walls of the deposition chamber, the temperature could reach near the softening point thereof, thus damaging the walls of film-forming chamber.
Specifically, for example, when aluminum is used for the walls of the film-forming chamber, temperatures near 450.degree. C. will deform screwing portions, portions under tensile stress, etc. to make them of no use.
In order to prevent such an accident, a high-melting-point material should be selected or a cooling means for preventing the increase of temperature in the walls of deposition chamber needs to be provided.
From the two problems described above, cooling of the walls of deposition chamber is necessitated as a significant technical subject in using the microwave discharge.
(Prior art about control of temperature of deposition chamber)
Japanese Patent Application Laid-open No. 1-36085 discloses a dry process apparatus for performing etching or deposition, provided with a cooling means for cooling the wall surfaces of deposition chamber (process chamber). It describes use of microwaves as a gas decomposing means. Further, it describes an improvement in cooling efficiency of the wall surfaces of process chamber by enclosing the process chamber in a vacuum vessel.
The above prior art, however, employs cooling of the process chamber walls for the purpose of preventing impurities deposited on the internal walls of the process chamber or the material forming the internal walls of process chamber from drifting away, and liquid nitrogen is listed as an example of a cooling agent. As apparent from this, there is nothing described about the problem of overcooling of the deposition chamber.
Japanese Patent Application Laid-open No. 60-24377 a lso discloses preventing degradation of film quality due to release gas by cooling the electrode and the internal walls of deposition chamber. It describes water, liquid nitrogen, and Freon as a cooling agent.
The prior art, however, describes that the temperature of the internal walls of a deposition chamber is desirably maintained at a temperature of not more than 150.degree. C., does not recognize the problem of overcooling, and describes nothing about maintaining the temperature of substrate around 300.degree. C. which usually forms a deposited film of good quality by .mu.W-CVD. When a deposited film is formed, especially on an elongated beltlike substrate as in the present invention, it is important to maintain the temperature of the substrate at a preferred value for a long time.
The walls of deposition chamber are held in the outer chamber for maintaining the pressure-reduced state as described above, and a cooling means under such reduced pressure is demanded.
The cooling means is conceivably one utilizing heat radiation.
This is a method for providing the deposition chamber and outer chamber with a plurality of heat radiation fins comprised of a plurality of blades disposed in an alternating arrangement and for cooling the walls of deposition chamber by mutual heat radiation and absorption between the fins. However, the cooling by heat radiation is low in cooling efficiency and is effective in low power, but does not achieve the effect as expected, in large power.
In consideration of the foregoing, we have repeated investigation on a method for utilizing heat conduction as a means for further raising the cooling efficiency.
This is a method for cooling the walls of deposition chamber by drawing a water-cooling pipe or the like into the outer chamber as vacuum-sealing it and keeping it in contact with the walls of deposition chamber.
As a result of our extensive and intensive investigation, it was found that good results could be achieved by maintaining the temperature of the walls of deposition chamber in the range of 100.degree. C. to 350.degree. C., preferably in the range of 150.degree. C. to 300.degree. C., while cooling the deposition walls.
The first reason is that temperatures of the walls of deposition chamber over 350.degree. C. will increase the temperature of the beltlike substrate as described above, to degrade the characteristics of a resulting solar battery.
The second reason is that temperatures of the walls of deposition chamber below 100.degree. C. will cause a phenomenon that it becomes difficult to maintain the microwave discharge, though the reason is indefinite.
On the other hand, it revealed another problem.
Since the cooling is direct cooling utilizing heat conduction, the cooling efficiency is too high, and it is not easy to maintain the temperature of the walls of deposition chamber in the range 150.degree. C. to 300.degree. C. for a long time, which decreases the temperature of the walls of deposition chamber. As a result, discharge is frequently interrupted as described above. If the amount of cooling water is reduced in order to raise the temperature of the walls of the deposition chamber, the cooling water will sometimes exceed the boiling point thereof to cause sudden volume expansion, which will break the water-cooling pipe.
This breakage of the water-cooling pipe would damage accessory parts, particularly, the vacuum pump or heaters, because of sudden flow of vapor into the vacuum vessel, or it would cause a dangerous accident such as gushing of hot water into the atmosphere.
From the above, a sufficient amount of water needs to flow so as to keep the temperature of cooling water below 100.degree. C. and the walls of deposition chamber need to be maintained at temperatures in the range of 150.degree. C. to 350.degree. C. as preventing overcooling thereof.
Moreover, consideration is necessitated not only on the cooling mechanism of the walls of the deposition chamber as described, but also on a heating mechanism.
(About baking)
At a degassing step, called baking, it is necessary to raise the temperature of the walls of the deposition chamber without help of discharge energy.
The temperature upon such baking is determined depending upon various factors including baking time, the wall material, desired film quality, and so on, but it is usually 100 or more .degree.C. and preferably not much lower than the temperature of the wall upon film formation.
Since the film-forming step is usually started after the baking step, it is desired to maintain the baking temperature closer to the temperature upon film formation from the aspect of stability of the temperature and characteristics in the initial stage of the film-forming step, and a sufficient temperature-raising mechanism is necessary.
It is, therefore, an object of the present invention to provide a film depositing method and film depositing apparatus using the microwave plasma enhanced CVD process that can mass-produce deposited films of stable quality over a long period by restraining temperature increases at the walls of deposition chamber forming the film-forming space, and by maintaining the temperature in the preferred range for deposition of film, thus solving the above problems in the prior art.